Kidney transplantation is the treatment of choice for kidney failure in children. Kidney transplant recipients need to take immunosuppression for the rest of their lives after transplantation to prevent rejection of the graft. One of the important medications is called Tacrolimus which can prevent rejection of the kidney graft but at the same time is toxic to the kidney. Therefore, repeated blood levels of Tacrolimus with tests of kidney function will help physicians to prescribe the best dose for therapy to prevent kidney rejection and kidney toxicity. Failure of compliance with taking Tacrolimus is also an important cause of graft failure especially among teenagers, so repeated blood Tacrolimus levels are necessary to detect patients who fail to take their medications on a regular basis. We have developed a lab assay that measures Tacrolimus blood level and creatinine (for kidney function) using one dried blood spot (DBS) on filter paper similar to the filter paper used in the new born screen. We plan to teach patients how to do the test at home once a month and mail the filter paper back to OHSU for analysis for Tacrolimus and creatinine. The advantage of this method is that it is less painful for children then a regular blood dray from the vein, can be done easily at home, will be most cost effective as it will save the family a day of work or school and can detect both the Tacrolimus level and the kidney function at the same time.